The most common conjunct that does not make position is "br", which regularly fails in this regard in the following words: "brāhmaṇ-", "brahma-", "brūti" (and its present declension), & "anubrūhaye". This last is particularly interesting because elsewhere "br" regularly does make position medially.

Other words that sometimes fail to make position are "tvaṁ", "dvāra", & "nhātaka".

"by" (or, alternatively "vy") quite often fails in this regard also. Other cases must remain doubtful.